Token auto-discovery, tx history, balance polling, EIP-6963, UI overhaul

Major changes:
- Fetch token balances and tx history from Blockscout API (configurable)
- Remove manual token discovery (discoverTokens) in favor of Blockscout
- HD address gap scanning on mnemonic import
- Duplicate mnemonic detection on wallet add
- EIP-6963 multi-wallet discovery + selectedAddress updates in inpage
- Two-tier balance refresh: 10s while popup open, 60s background
- Fix $0.00 flash before prices load (return null when no prices)
- No-layout-shift: min-height on total value element
- Aligned balance columns (42ch address width, consistent USD column)
- All errors use flash messages instead of off-screen error divs
- Settings gear in global title bar, add-wallet moved to settings pane
- Settings wells with light grey background, configurable Blockscout URL
- Consistent "< Back" buttons top-left on all views
- Address titles (Address 1.1, 1.2, etc.) on main and detail views
- Send view shows current balance of selected asset
- Clickable affordance policy added to README
- Shortened mnemonic backup warning
- Fix broken background script constant imports

src/shared/log.js

@@ -0,0 +1,30 @@
+// Leveled logger. Outputs to console with [AutistMask] prefix.
+// Level is DEBUG when the DEBUG constant is true, INFO otherwise.
+
+const { DEBUG } = require("./constants");
+
+const LEVELS = { debug: 0, info: 1, warn: 2, error: 3 };
+const threshold = DEBUG ? LEVELS.debug : LEVELS.info;
+
+function emit(level, method, args) {
+    if (LEVELS[level] >= threshold) {
+        console[method]("[AutistMask]", ...args);
+    }
+}
+
+const log = {
+    debugf(...args) {
+        emit("debug", "log", args);
+    },
+    infof(...args) {
+        emit("info", "log", args);
+    },
+    warnf(...args) {
+        emit("warn", "warn", args);
+    },
+    errorf(...args) {
+        emit("error", "error", args);
+    },
+};
+
+module.exports = { log };